It is basic to the collector's art to arrange items into groups. Postage stamps can be arranged by country of origin and then on face value, year of issue, design, colour variation, or defects. The arranging process always resolves into a hierarchic set of groups. In the plant kingdom we have a descending hierarchy of groups through Divisions, divided into Classes, divided into Orders, divided into Families, divided into Genera, divided into Species. Subsidiary groupings are possible at each level of this hierarchy and are employed to rationalize the uniformity of relationships within the particular group. Thus, a genus may be divided into a mini-hierarchy of subgenera, divided into sections, divided into series in order to assort the components into groupings of close relatives. All such components would, nevertheless, be members of the one genus.

Early systems of classification were much less sophisticated and were based upon few aspects of plant structure such as those which suggested signatures, and mainly upon ancient herbal and medicinal concepts. Later systems would reflect advances in man's comprehension of plant structure and function, and employ the morphology and anatomy of reproductive structures as defining features. Groupings such as Natural Orders and Genera had no precise limits or absolute parity, one with another; and genera are still very diverse in size, distribution and the extent to which they have been subdivided.

Otto Brunfels (1489–1534) was probably the first person to introduce accurate, objective recording and illustration of plant structure in his Herbarium of 1530, and Valerius Cordus (1515–1544) could have revolutionized botany but for his premature death.